Dear jobphys,
limb length discrepancy is defined as a 2.5 cm (1inch) or greater leg length. Differences that are smaller tend not to cause clinical problems.
If your childs limb length is more(or becomes more) than 1 inch I suggest having this checked. It could have some underlying reason (congenital, neuromuscular,infections, aquired, vascular etc)
If you are a fysio, you can check the ROM, joint stability, muscle strength, of trunk, hips, knees and ankles. Check sensibility and functional activities, posture and gait. If you are unsure and if you have the impression the leg difference is increasing go to a paediatrician.
kind regards and good luck to you both
Esther